High stability electron beam generator for processing material

ABSTRACT

Apparatus for generating and controlling an electron gun in a vacuum envelope comprising, a cathode, means for controlling the temperature of the cathode to control its current emission, an auxiliary electrode with an aligned opening mounted adjacent to said cathode and with the electron beam from the cathode passing through said aligned opening, means for maintaining the voltage of said auxiliary electrode slightly more negative than said cathode, and an annular anode mounted in a spaced relationship from said cathode and said auxiliary electrode.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to a high stability electron beam generator forprocessing material, comprising an electrically heated cathode, aperforated anode, and a control electrode lying at a potential which ismore positive than the cathode and disposed between the cathode and theanode.

2. Underlying Prior Art

In the known electron beam generating system for processing material,the electron beam is generated and controlled by three electrodes,cathode, perforated anode and Wehnelt cylinder. Directly or indirectlyheated H-needle, ribbon- or bolt-cathodes consisting of tungsten areemployed as the cathode and cathodes consisting of lanthanhexaborate arealso employed.

The electrons of the electron beam are emitted from the cathode due tothe heating and are subsequently accelerated. For example, the cathodecan be charged with a high negative voltage, whereas the anode lies atgrounded potential. The electrodes are accelerated toward the anode inthe electrical field generated in that manner, pass through the anodeperforation and arrive in the field-free space.

In the previous systems, the third electrode, the Wehnelt or latticeelectrode, serves for controlling the current of the beam. The functionof the Wehnelt electrode is described, for example, in the publicationM. Blocke, Zeitschrift fur angewandte Physik, Vol. 3, 1951, pp. 441through 449, "Elementare Theorie der Elektronenstrahlerzeugung mitTrioden-systemen".

When the negative voltage of the Wehnelt electrode is increased beyond aspecific value, then further electrons are incapable of leaving thecathode surface, i.e., there are no electrical field vectors whichaccelerate the electrons to the anode directed to the anode at thecathode surface. The emission is blocked. When the Wehnelt voltage isreduced, then the anode penetration coefficient increases. Given adecreasing Wehnelt voltage, larger and larger areas of the cathode arereleased for emission, whereby the emission current increases. Thus, thevariable negative voltage of the Wehnelt cylinder in comparison to thecathode allows a control of the beam current.

A characteristic of these systems is described in greater detail below.It derives from the structural disposition of the Wehnelt cylinder anddue to the prescribed Wehnelt voltage which is required for setting adesired beam current, a fixed beam geometry which can only be changed bymeans of changing the structural shape or the Wehnelt voltage. However,a change of the Wehnelt voltage again leads to a change of the beamcurrent. This rigid dependency of the beam current upon the Wehneltvoltage and the beam shape fixed by the Wehnelt arrangement and Wehneltvoltage are disadvantageous.

Added thereto as a further disadvantage is that, in the standardoperating mode, the cathode is operated in the space charge range,whereby the beam value lies below the value which would theoreticallyderive given operation of the cathode in the saturation range. Resultingtherefrom is that the efficiency of the electron beam generator does notassume the optimally attainable value.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to create anelectron beam generator which does not exhibit these disadvantages andwhich has a higher beam value in comparison to known electron beamgenerators.

Thus, the invention consists of an electron beam generating system witha heated cathode, a perforated anode, and an auxiliary electrode lyingat a more negative potential than the cathode, and is characterized inthat the beam current is determined by means of rating the activesurface of the cathode and by means of the controlled heating of thecathode to a constant temperature.

An advantageous further development consists therein that the auxiliaryelectrode surrounds the cathode and lies at a slightly more negative,constant potential than the cathode. In an advantageous manner, theauxiliary electrode is designed as a perforated disk surrounding thecathode. According to an advantageous further development, the structureis undertaken in such manner that the surface of the auxiliary electrodepointing toward the anode aligns with the surface of the cathodepointing toward the anode. Other objects, features and advantages of theinvention will be readily apparent from the following description ofcertain preferred embodiments thereof taken in conjunction with theaccompanying drawings although variations and modifications may beeffected without departing from the spirit and scope of the novelconcepts of the disclosure, and in which:

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described in greater detail on thebasis of FIGS. 1 through 3. There are shown:

FIG. 1 an example of the disposition of the electrodes of the electronbeam generating system and of the beam geometry of the electron beam;

FIG. 2 a further example of the beam geometry of the electron beam; and

FIG. 3 a circuit arrangement for heating the cathode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a schematically illustrated cathode 1 is surrounded by anannular auxiliary electrode 2. The auxiliary electrode 2 lies at asomewhat more negative potential than the cathode 1. When the cathode 1is heated, then it predominantly emits an electron beam 4 from itssurface 11, said electron beam 4 moving in the direction of a perforatedanode 3 and passing through the anode perforation 31.

Let approximately the following voltages be adjacent to the individualelectrodes: cathode -50 KV, auxiliary electrode -50.5 KV and anode 0 V.

An equipotential surface of the potential of the cathode surface isreferenced with the reference numeral 22, and one can see that itslightly increases proceeding from the edge 12 of the cathode surface 11in order to again assume a constant progress across the auxiliaryelectrode 2. What is thereby achieved is that no electrons which emergefrom the lateral surfaces of the cathode contribute to the beam.

In conjunction with the heating of the cathode to a constanttemperature, an optimum power exploitation of the system is achieved bymeans of this disposition of cathode and auxiliary electrode, wherebythe influence of the auxiliary electrode 2 is additionally usable as afree parameter for the beam shaping, this to be explained in greaterdetail below.

It is possible to operate the cathode in saturation by means of thepotential path indicated in FIG. 1. All electrons are then directlysuctioned from the cathode surface, since the potential differencebetween the cathode and the anode generates an electrical field whichbecomes effective at the cathode surface nearly without attenuation.

Unlike known arrangements, a space charge zone is not formed above thecathode. The reasons for this space charge zone in known arrangementslies therein that, given a dimensioning of the emission surface of thecathode which is not precise, it is necessary to apply a relatively highnegative potential to the Wehnelt electrode for the control of the beamcurrent. Thereby, the draw-off field strength in front of the cathode isgreatly reduced. In this case, a space charge zone consisting ofelectrons is formed in front of the cathode. The anode-side surface ofthis space charge zone now functions as the actual electron source.Since the emission surface of the space charge zone is always greaterthan the active surface of the cathode itself, the emission density inthese systems is lower, i.e., the beam value is poorer.

FIG. 2 shows an example as to how, given retention of the optimumexploitation of the beam energy, an altered shaping of the beam ensuesby means of the auxiliary electrode 2. Since FIG. 2 is a matter of thesame structural parts as in FIG. 1, the same reference numerals havebeen employed. In contrast to FIG. 1, the potential of the auxiliaryelectrode 2 has been selected somewhat more negative, namely, at -51 KV,for example. One can see that the curve of the -50 KV line 22 betweenthe cathode and auxiliary electrode proceeds more steeply than inFIG. 1. This leads to the fact that the electrons emerging from thecathode are focused into a slimmer electron beam.

Thus, by means of setting the voltage of the auxiliary electrode, it ispossible in a simple manner to alter the beam geometry of the electronbeam without the current intensity being altered.

In saturation mode, the emission current of a cathode surface of aspecific size only depends on the cathode temperature. In order, thus,to set a specific current intensity of the electron beam in the presentcase, the cathode heating power is regulated in such manner that thecathode constantly exhibits the temperature appertaining to the desiredcurrent intensity.

A schematic circuit for holding the cathode temperature constant given adirectly heated cathode is specified in FIG. 3.

The emission current i_(E) flows from the high voltage generator Hacross the current precision resistor RM to the cathode K. The voltageUE proportional to the emission current drops off at the precisionresistor. A voltage US proportional to the desired beam current isgenerated in the rated value generator S. The voltages US and UE aresupplied to the regulator R. The regulator R sets the voltage source SQof the heater circuit to the cathode K in such manner that UE becomesUE=US, i.e., the cathode emits the constant current i_(E), whereby aconstant cathode temperature is observed because of the clearinterrelationship between the emission current and the cathodetemperature.

A further advantage of the invention becomes clear as the result of acomparison to a traditional system:

Since, in a traditional system, the beam current is controlled by meansof the Wehnelt voltage, the heating capacity must be set in such mannerthat the cathode assumes a higher temperature than would be necessaryfor attaining the desired beam current. However, the cathode resistanceRK varies during the useful life of the cathode due to evaporation ofcathode material. When the cathode is heated with constant current, thenthe cathode heating capacity increases and, thus, the cathodetemperature constantly increases according to the equation forconstant-current heating capacity N_(H)

    N.sub.H =i.sub.H.sup.2 ·R.sub.K

Accordingly, the cathode temperature decreases with the heating capacitygiven constant-voltage heating according to the equation

    N.sub.H =U.sub.H.sup.2 /RK

The change of the cathode temperature as the result of aging, however,causes a change of the emission efficiency of the cathode. A constantbeam current can be observed by means of controlling the Wehneltvoltage, but, at the same time, the necessary changes of the Wehneltvoltage effect a change of the beam geometry.

These disadvantages are avoided in the inventive arrangement since theWehnelt voltage once set in order to achieve a favorable beam shape isnot altered, since the heating capacity is controlled during thelife-expectancy of the cathode in such manner that the cathode alwaysexhibits the same temperature.

Commercial Utilization

The invention can be advantageously employed in processing material withelectron beams in which high energy density and a precise beam shape area matter of concern.

A further area of application lies in the production of printing formsby means of an electron beam, whereby the cups required for the printingoperation are engraved from the surface of the printing form with theelectron beam.

Moreover, the invention can be applied in electron beam microscopy.Although the invention has been described with respect to preferredembodiments, it is not to be so limited as changes and modifications canbe made which are within the full intended scope of the invention asdefined by the appended claims.

I claim:
 1. An electron beam generating system comprising a heatedcathode, a perforated anode spaced from said cathode, an auxiliaryelectrode surrounding the cathode and the electron beam shape and thebeam intensity are determined because a slightly more negative constantvoltage is applied to the auxiliary electrode than is applied to thecathode, means for controlling the heating of the cathode to a constanttemperature, including means for measuring the actual emission currentof the cathode and supplying an output to said means for controlling theheating current of the cathode.
 2. An electron beam generating systemaccording to claim 1 wherein said auxiliary electrode is a perforateddisk which surrounds said cathode.
 3. An electron beam generatoraccording to claim 2 wherein the planar surface of said cathode facingsaid anode is aligned with the planar surface of the auxiliary electrodewhich faces said anode.
 4. Apparatus for generating and controlling anelectron gun in a vacuum envelope comprising, a cathode with an activeend surface and a heater, an annular anode mounted in a spacedrelationship from said cathode, an auxiliary electrode with an alignedopening mounted adjacent to said cathode and with the electron beam fromthe cathode controlled by said auxiliary electrode, means formaintaining the voltage of said auxiliary electrode slightly morenegative than said cathode, means for controlling the heating current ofthe cathode to a constant temperature connected to the heater of saidcathode, means for measuring the actual emission current of the cathodeconnected to supply an output to said means for controlling the heatingcurrent of the cathode.
 5. Apparatus according to claim 4 wherein saidauxiliary electrode comprises a planar disk member with said alignedopening surrounding the electron surface of said cathode.
 6. Apparatusaccording to claim 5 including a high voltage generator, said means formeasuring the actual emission current of the cathode comprising acurrent precision resistor connected to said high voltage generator andto said cathode and measuring the emission current of said cathode, aregulator connected to said current precision resistor, a variablevoltage source connected to said regulator to supply a voltageproportional to the desired beam current, and a voltage source connectedto said cathode and receiving an input from said regulator forcontrolling the heating current of said cathode.